If they haven't sold (or given away) the rights.  Copyright can only be abandoned by an explicit act of the owner, not by mere neglect.

Very unlikely (which is lawyerese for "Not a chance").  The terms of the AT&T master license to Sun aren't public knowledge, but it probably limited Sun to distributing Solaris 2.0+ in binary form (with the usual exceptions around contractors, etc.).  To distribute Solaris in source form would require Sun to license the rights needed to do so from the copyright owner.

It's not clear to me just who Sun licensed them from, thanks to the Novell-SCO dispute.  At any rate, Sun got what they considered sufficient title for the Solaris 11 release under the CDDL  But that would not allow any other licensee of AT&T or its successors in title to do the same thing without a separate license from the owner.  Whatever the precise terms of the Sun-Novell license, it would grant rights to Sun and nobody else.

If Acme Films licenses the right to make a movie of _Passionate Unix_, a book owned by Yoyodyne Publishing, then another movie licensee of Yoyodyne wouldn't get the rights, based on *their* movie license, to publish the original book.  (In practice Acme would insist that Yoyodyne not license the movie rights to anyone else.)

IANAI; TINLA.